Polythiophene-containing ink compositions for inkjet printing

ABSTRACT

Ink compositions comprising polythiophenes and methicone that are formulated for inkjet printing the hole injecting layer (HIL) of an organic light emitting diode (OLED) are provided. Also provided are methods of inkjet printing the HILs using the ink compositions.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional patent applicationNo. 61/898,343, entitled Polythiophene-Containing Ink Compositions ForInkjet Printing, which was filed on Oct. 31, 2013, the entire contentsof which are incorporated herein by reference. This application is acontinuation-in-part of U.S. patent application Ser. No. 13/618,157,entitled Film-Forming Formulations for Substrate Printing, which wasfiled on Sep. 14, 2012, and which claims priority to U.S. provisionalpatent application No. 61/535,413, which was filed on Sep. 16, 2011, theentire contents of which are incorporated herein by reference.

BACKGROUND

Ink compositions for inkjet printing layers in organic light emittingdiodes (OLEDs) have been proposed. However, problems associated withinadequate wetting properties of the ink compositions has stifled thedevelopment of printable inks because improper wetting leads tonon-uniform film formation and, therefore, non-uniform luminescence fromorganic light emitting diode pixels that incorporate the printed films.Another challenge that has impeded the development of inkjet printablecompositions for OLED applications is the inability to incorporate highconcentrations of active polymers into the inks, while maintaining ajettable ink formulation.

SUMMARY

Ink compositions comprising polythiophenes that are formulated forinkjet printing the hole injecting layer (HIL) of an OLED are provided.Some embodiments of the ink compositions are characterized by theinclusion of a methicone as a pinning agent. Others are characterized bythe inclusion of aprotic solvents that enable the incorporation of highconcentrations of the polythiophene in the inks. Also provided aremethods of inkjet printing HILs using the ink compositions.

One embodiment of a method of forming an HIL for an organic lightemitting diode comprises the steps of: inkjet printing a droplet (thatis, at least one droplet) of an ink composition over an electrode layerin a pixel cell of an organic light emitting diode, the pixel celldefined by a pixel bank; and allowing the volatile components of the inkcomposition to evaporate, whereby the hole injecting layer is formed. Anembodiment of an ink composition that can be used in the methodcomprises: an electrically conductive polythiophene; water; at least oneorganic solvent; and methicone, wherein the methicone is present in anamount that provides contact line pinning of the droplet in the pixelcell.

Some embodiments of the ink compositions comprise:poly(3,4-ethylenedioxythiophene); water; at least one organic solventhaving a surface tension no greater than 55 dyne/cm at 25° C., aviscosity no greater than 15 cPs at 25° C., and a boiling point of atleast 200° C.; and methicone. The at least one organic solvent can be,for example, sulfolane.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1 is a block diagram that illustrates an OLED inkjet printingsystem.

FIG. 2 is a schematic representation of gas enclosure system that canhouse the printing system shown in FIG. 1

FIG. 3 is a schematic illustration of a flat panel display comprising aplurality of OLEDs arranged in a matrix of pixel cells, each pixel cellbeing defined by a pixel bank.

FIG. 4A is a microphotograph image of an ink composition containing 0.08wt. % methicone pinned in an OLED pixel cell.

FIG. 4B is a black and white line drawing of the microphotograph in FIG.4A

FIG. 5A is a microphotograph image of an ink composition that is free ofmethicone spilling over an OLED pixel cell.

FIG. 5B is a black and white line drawing of the microphotograph in FIG.5A.

FIG. 6A is a microphotograph image of an ink composition that is free ofmethicone dewetting an OLED pixel cell.

FIG. 6B is a black and white line drawing of the microphotograph in FIG.6A.

FIG. 7A is a microphotograph of the luminescence emitted from an OLEDpixel having an HIL printed with an ink composition comprising methiconeas a pinning agent.

FIG. 7B is a black and white line drawing of the microphotograph in FIG.7A.

FIG. 8A is a microphotograph of the luminescence emitted from OLEDpixels in which the HILs were printed with an ink composition comprisingmethicone as a pinning agent and sulfolane as an organic solvent.

FIG. 8B is a black and white line drawing of the microphotograph in FIG.8A.

FIG. 9A is a microphotograph of the luminescence emitted from OLEDpixels in which the HILs were printed with an ink composition comprisingmethicone as a pinning agent and 1,3-propanediol as an organic solvent.

FIG. 9B is a black and white line drawing of the microphotograph in FIG.9A.

FIG. 10 is a graph of drop volume over time for an ink compositionbefore and after a 30 minute idle of the inkjet printing nozzle, asdescribed in Example 2.

FIG. 11 is a graph of drop velocity over time for an ink compositionbefore and after a 30 minute idle of the inkjet printing nozzle, asdescribed in Example 2.

FIG. 12 is a graph of drop angle over time for an ink composition beforeand after a 30 minute idle of the inkjet printing nozzle, as describedin Example 2.

DETAILED DESCRIPTION

Ink compositions comprising polythiophenes that are formulated forinkjet printing the HIL of an OLED are provided. Also provided aremethods of inkjet printing the HILs using the ink compositions.

The ink compositions are characterized by high concentrations ofelectrically conducting polythiophenes, such aspoly(3,4-ethylenedioxythiophene) (PEDOT), yet provide wetting, jettingand latency properties that render them well-suited for inkjet printingonto pixilated substrates, such as OLED pixel cells. In addition, theink compositions provide printed HILs having highly uniform thicknessesand homogenous compositions. As a result, the printed HILs contribute toa highly uniform light emission profile for OLEDs into which they areincorporated. The enhanced printability provided by the ink compositionscan be attributed, at least in part, to the realization that, at anappropriate concentration, methicone can act as a contact line pinningagent for droplets of the ink compositions in a pixel cell. By providingcontact line pinning, the methicone ensures that the footprint of adroplet of the ink composition deposited into a pixel cell remainsunchanged from its initial form during the process of drying.

A basic embodiment of the ink compositions is an aqueous solutioncomprising an electrically conductive polythiophene, methicone, at leastone organic solvent and water. A basic embodiment of a method of formingan HIL for an OLED using one of the ink compositions comprises the stepsof depositing a droplet of the ink composition over a layer ofelectrically conductive material (i.e., an anode) in a pixel cell of anorganic light emitting diode array and allowing the volatile componentsof the ink composition to evaporate, leaving a solid HIL. The step ofallowing the volatile components (e.g., water and organic solvents) toevaporate may be facilitated by subjecting the printed ink compositionto reduced pressure, that is—exposing it to a vacuum, by exposing theprinted ink composition to elevated temperatures, or a combination ofthe two.

Methicones are silicone oils polymerized from siloxanes. They are alsoreferred to as methyl hydrogen siloxanes or methyl siloxanes. Methiconesare commercially available and sold as surfactants by Botanigenics(Northridge, Calif.) under the tradename Botanisil®. These includeBotanisil® AD-13, AM-14, ATC-21, BPD-100, CD-80, CD-90, CE-35, CM-12,CM-13, CM-70, CP-33, CPM-10, CS-50, CTS-45, DM-60M, DM-85, DM-90, DM-91,DM-92, DM-93, DM-94, DM-95, DM-96, DM-97, DTS-13, DTS-35, GB-19, GB-20,GB-23, GB-25, GB-35, L-23, ME-10, ME-12, PSS-150, PT-100, S-18, S-19,S-20, TSA-16, and TSS-1. Methicones are also available from the LubrizolCorporation (Wickliffe, Ohio) under the tradename SilSense®. Theseinclude SilSense® Copolyol-1 Silicone (PEG-33 (and) PEG-8 Dimethicone(and) PEG 14), SilSense® DW-18 Silicone (Dimethicone PEG-7 Isostearate),SilSense® SW-12 Silicone (Dimethicone PEG-7 Cocoate), SilSense® IWS(Dimethiconol Ester Dimethiconol Stearate), SilSense® A-21 Silicone(PEG-7 Amodimethicone), SilSense® PE-100 Silicone (Dimethicone PEG-8Phosphate), and Ultrabee™ WD Silicone (Dimethicone PEG-8 Beeswax).

In the present ink compositions, the amount of methicone is carefullycontrolled, such that the methicone acts as a contact line pinningagent. This is important because it prevents the pinned ink compositiondroplets from pulling away from portions of the banks of the pixel cell(dewetting), which is sometimes accompanied by over-spill at otherportions of the pixel cell. It also prevents the ink compositions frompiling up at the sides of, or spreading beyond, the pixel cells, aswould happen with more complete wetting.

The ink compositions can be used to form HILs on a variety of OLEDelectrode materials. Most commonly, the electrode substrate willcomprise a transparent electrically conductive material, such as atransparent conductive oxide (TCO) or silicon. The appropriateconcentration range for methicone in an ink composition will depend onthe nature of the underlying substrate. However, for a given substrate,the concentration range at which the methicone provides contact linepinning can be determined by observing the wetting behavior of inkdroplets having different methicone concentrations that have beenapplied to the surface via a drop casting process. By way ofillustration, some embodiments of the present ink compositions comprisemethicone in an amount of no greater than 0.15 weight percent (wt. %),no greater than 0.12 wt. % or no greater than 0.1 wt. %, based on thetotal weight of the ink composition. This includes embodiments of theink compositions in which methicone is present in an amount in the rangefrom 0.02 to 0.15 wt. %, further includes embodiments in which methiconeis present in an amount in the range from 0.03 to 0.12 wt. %, and stillfurther includes embodiments in which methicone is present in an amountin the range from 0.05 to 0.1 wt. %, based on the total weight of theink composition. Such ranges are suitable when the HIL ink compositionsare printed onto known anode materials used in OLED devices. Forexample, in the case of an OLED device where the light is emittedthrough the anode (termed bottom emitting), a transparent or translucentanode material is used. Transparent or translucent anode materials caninclude indium oxide, zinc oxide, indium tin oxide (ITO), and indiumzinc oxide (IZO) or the like. In the case of an OLED device where thelight is emitted through the cathode (termed top emitting), a reflectivelayer is formed below the transparent anode. Reflective layer materialscan include silver (Ag), silver-palladium-copper (APC),silver-rubidium-gold (ARA), molybdenum-chromium (MoCr) or the like.

The aqueous ink compositions further include one or more electricallyconducting polythiophenes. For example, PEDOT and mixtures of PEDOT withpoly(styrenesulfonate) (PEDOT:PSS) may be included in the inkcompositions. Notably, in combination with appropriate solvents, asdiscussed in greater detail below, the polythiophenes can be included inthe ink compositions at very high concentrations. For example, someembodiments of the ink compositions comprise at least 30 wt. %polythiophene, at least 40 wt. % polythiophene, at least 50 wt. %polythiophene, at least 55 wt. % polythiophene, or at least 60 wt. %polythiophene, based on the total weight of the ink composition. In suchembodiments, the polythiophene can be PEDOT.

The aqueous ink compositions comprise at least one organic solvent. Forexample, a composition may comprise a solvent that reduces the surfacetension and/or viscosity of the composition, a solvent that increasesthe latency of the printed ink composition, or a combination of thesetypes of solvents. The at least one organic solvent can be a solventhaving a relatively high boiling point that increases the latency of theprinted ink compositions. This is advantageous because it helps toprevent the ink compositions from drying onto and clogging print nozzlesduring printing. Such solvents desirably have boiling points of at least200° C. More desirably they have boiling points of at least 230° C., atleast 250° C. or even at least 280° C. Diols and glycols, such aspropanediols, pentanediols, diethylene glycols and triethylene glycols,are examples of organic solvents that can be used to increase latency.Unfortunately, however, the diols and glycols tend to have relativelyhigh viscosities and surface tensions, which can degrade the jettabilityof ink compositions that include them. Therefore, some embodiments ofthe present ink compositions are free of diol and glycol solvents. Inthese embodiments, aprotic solvents having boiling points of at least240° C., viscosities of no greater than 15 cPs and surface tensions ofno greater than 55 dyne/cm, can be used instead of diols or glycols.This includes aprotic solvents having viscosities of no greater and 12cPs and further includes those having viscosities of no greater than 10cPs. For the purposes of this disclosure, the recited boiling pointsrefer to boiling points at atmospheric pressures. The recitedviscosities and surface tensions refer to viscosities and surfacetensions at the printing temperature. For example, if printing occurs atroom temperature, the viscosities and surface tensions will be those atabout 25° C.

Sulfolane, 2,3,4,5-tetrahydrothiophene-1,1-dioxide, also known astetramethylene sulfone, is an example of a relatively high boiling,relatively lower viscosity aprotic solvent that provides good latencywithout sacrificing jettability. Moreover, ink compositions that includesulfolane as an organic solvent can incorporate high concentrations ofboth the solvent and the polythiophene, while maintaining goodjettability. For example, the ink compositions may comprise sulfolane inamounts of at least 5 wt. %, at least 10 wt. % or at least 12 wt. %.Suitable concentration ranges for the sulfolane in the ink compositionsinclude the range from about 3 wt. % to about 15 wt. %. At thesesulfolane concentrations, the ink compositions can incorporate highconcentrations of PEDOT (e.g., 35 to 70 wt. %). In some of the inkcompositions, sulfolane is the majority solvent, that is, it makes upgreater than 50 wt. % of the total organic solvent content of the inkcomposition. Other suitable solvents include propylene carbonate and1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, also known asdimethylpropylene urea.

The ink compositions can further include a co-solvent that acts as asurface-tension reducer in order to enhance the jettability of thecomposition. For example, ink compositions comprising diols, glycols,sulfolane or other high boiling point solvents may include an additionalsolvent having a lower surface tension and, typically, a lower boilingpoint, than those solvents. Propylene glycol methyl ethers or othersimilar ethers may be used for this purpose.

Generally, for ink compositions useful for inkjet printing applications,the surface tension, viscosity, latency and wetting properties of theink compositions should be tailored to allow the compositions to bedispensed through an inkjet printing nozzle without drying onto orclogging the nozzle at the temperature used for printing (e.g., roomtemperature; ˜25° C.). Thus, the optimal properties will vary dependingupon such factors as nozzle dimensions, printing speed and printingtemperature. Generally, acceptable viscosities will include those in therange from about 1 to about 20 cPs and acceptable surface tensions willinclude those below about 50 dynes/cm. In order to eliminate or minimizenozzle clogging, latencies of 20 minutes or longer (e.g., 30 minutes orlonger) (at room temperature and without vacuum) are desirable, wherelatency refers to the time that nozzles can be left uncovered and idlebefore there is a significant reduction in performance, for instance areduction in drop velocity that will noticeably affect the imagequality.

Inkjet printers suitable for printing the ink compositions arecommercially available and include drop-on-demand printheads, availablefrom, for example, Fujifilm Dimatix (Lebanon, N.H.), TridentInternational (Brookfield, Conn.), Epson (Torrance, Calif.), HitachiData systems Corporation (Santa Clara, Calif.), Xaar PLC (Cambridge,United Kingdom), and Idanit Technologies, Limited (Rishon Le Zion,Isreal) and Ricoh Printing Systems America, Inc. (Simi Valley, Calif.).For example, the Dimatix Materials Printer DMP-3000 may be used.

As depicted in the block diagram of FIG. 1, various embodiments of OLEDinkjet printing system 100 can be comprised of several devices,apparatuses and systems and the like, which allow the reliable placementof ink drops onto specific locations on a substrate. According tovarious embodiments of systems and methods, a printing system caninclude, for example, but not limited by, a substrate conveyance system110, a substrate support apparatus 120, a motion system 130, a printheadassembly 140, an ink delivery system 150, and a control system 160.

An OLED substrate can be inserted and removed from printing system 100using a substrate conveyance system 110. Depending on variousembodiments of printing system 100, substrate conveyance system 110 canbe a mechanical conveyor, a substrate floatation table with a gripperassembly, a robot with end effector, and combinations thereof.Additionally, during a printing process, a substrate can be supported bysupport apparatus 120, which can be, for example, but not limited by, achuck or a floatation table. As printing requires relative motionbetween the printhead and the substrate, various embodiments of printingsystem 100 can have motion system 130, which can be, for example, butnot limited by, a gantry or split axis XYZ system.

Printhead assembly 140 can include at least one printhead device thatcan be mounted to motion system 130. The at least one printhead deviceincluded in printhead assembly 140 can have at least one inkjetprinthead capable of ejecting drops of an ink composition at acontrolled rate, velocity, and size through at least one orifice.Various embodiments of printing system 100 according to the presentteachings can have between about 1 to about 60 printhead devices.Additionally, various embodiments of a printhead device can have betweenabout 1 to about 30 inkjet printheads in each printhead device, whereeach inkjet printhead can have between about 16 to about 2048 nozzles.According to various embodiments of printhead assembly 140, each nozzleof each inkjet printhead can expel a drop volume of between about 0.1 pLto about 200 pL. Printhead assembly 140 with at least one inkjetprinthead can be in fluid communication with an ink composition deliverysystem 150, which can supply an ink composition to one or more inkjetprintheads of printhead assembly 140.

Regarding various embodiments of motion system 130, during a printingprocess, either printhead assembly 140 can move over a stationarysubstrate (gantry style), or both printhead assembly 140 and a substratecan move, in the case of a split axis configuration. For variousembodiments of a split axis configuration, Z axis control can beprovided by moving printhead assembly 140 relative to a substrate. Instill another embodiment of a motion system, printhead assembly 140 canbe fixed, and a substrate can move in the X and Y axes relative toprinthead assembly 140, with Z axis motion provided either by Z-axismovement of printhead assembly 140 or by Z-axis movement of a substrate.During a printing process, as printhead assembly 140 moves relative to asubstrate, drops of an ink composition are ejected at the correct timeto be deposited in the desired location on a substrate.

For various embodiments of printing system 100, control system 160 canbe used to control the functions of the printing process. Variousembodiments of control system 160 can be accessible to an end userthrough a user interface. Control system 180 can be used to control,send, and receive data to and from substrate conveyance system 110,substrate support apparatus 120, motion system 130, printhead assembly140, and an ink composition delivery system 150. Control system 160 canbe a computer system, a microcontroller, an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), anelectronic circuit capable of sending and receiving control and datainformation and capable of executing instructions, and combinationsthereof. Control system 160 can include one electronic circuit ormultiple electronic circuits distributed among substrate conveyancesystem 110, substrate support apparatus 120, motion system 130,printhead assembly 140, and an ink composition delivery system and 150for the purpose of providing communication between components, forexample.

Additionally, for various embodiments of control system 160 of printingsystem 100 can provide data processing, display and report preparationfunctions. All such instrument control functions may be dedicatedlocally to printing system 100, or control system 160 can provide remotecontrol of part or all of the control, analysis, and reportingfunctions. Finally, various embodiments of printing apparatus 100 can behoused in enclosure 200 of FIG. 2.

FIG. 2 is a schematic representation of gas enclosure system 200 thatcan house printing system 100 of FIG. 1, in accordance with variousembodiments. Various embodiments of a gas enclosure system 200 cancomprise a gas enclosure assembly 250 according to the presentteachings, a gas purification loop 230 in fluid communication with gasenclosure assembly 250, and at least one thermal regulation system 240.Additionally, various embodiments of a gas enclosure system can havepressurized inert gas recirculation system 260, which can supply inertgas for operating various devices, such as a substrate floatation tablefor an OLED printing system. Various embodiments of a pressurized inertgas recirculation system 260 can utilize a compressor, a blower andcombinations of the two as sources for various embodiments of inert gasrecirculation system 260. Additionally, gas enclosure system 200 canhave a filtration and circulation system (not shown) internal to gasenclosure system 200, which along with other components, such as afloatation table, can provide a substantially low-particle printingenvironment.

As depicted in FIG. 2, for various embodiments of gas enclosure assembly200 according to the present teachings, gas purification loop 230 caninclude outlet line 231 from gas enclosure assembly 250, to a solventremoval component 232, and then to gas purification system 234. Inertgas purified of solvent and other reactive gas species, such as oxygenand water vapor, are then returned to gas enclosure assembly 250 throughinlet line 233. Gas purification loop 230 may also include appropriateconduits and connections, and sensors, for example, oxygen, water vaporand solvent vapor sensors. A gas circulating unit, such as a fan, bloweror motor and the like, can be separately provided or integrated, forexample, in gas purification system 234, to circulate gas through gaspurification loop 230. According to various embodiments of a gasenclosure assembly, though solvent removal system 232 and gaspurification system 234 are shown as separate units in the schematicshown in FIG. 2, solvent removal system 232 and gas purification system234 can be housed together as a single purification unit. Thermalregulation system 240 can include, for example, but not limited by, atleast one chiller 241, which can have fluid outlet line 243 forcirculating a coolant into a gas enclosure assembly, and fluid inletline 245 for returning the coolant to the chiller.

For various embodiments of gas enclosure assembly 200, a gas source canbe an inert gas, such as nitrogen, any of the noble gases, and anycombination thereof. For various embodiments of gas enclosure assembly200, a gas source can be a source of a gas such as clean dry air (CDA).For various embodiments of gas enclosure assembly 200, a gas source canbe a source supplying a combination of an inert gas and a gas such asCDA.

Gas enclosure system 200 can maintain levels for each species of variousreactive gas species, including various reactive atmospheric gases, suchas water vapor and oxygen, as well as organic solvent vapors at 100 ppmor lower, for example, at 10 ppm or lower, at 1.0 ppm or lower, or at0.1 ppm or lower. Further, various embodiments of a gas enclosureassembly can provide a low particle environment meeting a range ofspecifications for airborne particulate matter according to ISO 14644Class 1 through Class 5 clean room standards.

While what is given in the above is an exemplary OLED inkjet printsystems and gas enclosure systems, one skilled in the art can appreciatethat such systems can be built with any combination of one or more ofthe devices and apparatuses of FIG. 1 and FIG. 2 as well as additionaldevices and apparatuses.

The final inkjet-printed product is an HIL having a highly uniformthickness and composition. For example, layers having a thicknessvariation no greater than 10% across the entire width of the layer arepossible. The thickness across the layer can be measured using metrologytools, such as a stylus contact profilometer or an interferometermicroscope. Suitable interferometers for optical interferometry arecommercially available from Zygo instrumentation.

The ink compositions can be used to print the HILs directly in amulti-layered OLED architecture. A typical OLED comprises a supportsubstrate, an anode, a cathode, a HIL disposed over the anode and alight-emitting layer (EML) disposed in between the HIL and the cathode.Other layers that may be present in the device, include a holetransporting layer provided between the HIL and the light-emitting layerto assist with the transport of holes to the light-emitting layer, andan electron transporting layer (ETL) disposed between the EML and thecathode. The substrate is generally a transparent glass or plasticsubstrate.

In these multi-layered architectures, one or more layers in addition tothe HIL may be formed via inkjet printing, while other layers may bedeposited using other film-forming techniques. Typically, the variouslayers will be formed within one or more pixel cells. Each pixel cellcomprises a floor and is defined by a bank that defines the perimeter ofthe cell. The surfaces within a cell optionally may be coated with asurface-modifying coating, such as a surfactant. However, in someembodiments, such surfactants are absent, as they may quench theluminescence of the light-emitting layer.

FIG. 3 is a schematic illustration of a flat panel display comprising aplurality of OLEDs arranged in a matrix of pixel cells. FIG. 3 depictsan expanded view 320 of an area of panel 300, showing the arrangement330 of a plurality of pixel cells, including a red light-emitting pixelcell 332, a green light-emitting pixel cell 334 and blue light-emittingpixel cell 336. Additionally, integrated circuitry 338 can be formed ona flat panel display substrate so that the circuitry is adjacent to eachpixel cell for the purpose of applying voltage to each pixel in acontrolled fashion during use. Pixel cell size, shape, and aspect ratioscan vary depending on, for example, but not limited by, the resolutiondesired. For example, a pixel cell density of 100 ppi can be sufficientfor a panel used for a computer display, where for high resolution of,for example of between about 300 ppi to about 450 ppi, can result invarious pixel cell designs amenable to the effective packing of higherpixel density on a substrate surface.

While the disclosure above has focused on aqueous ink compositionsformulated for inkjet printing polythiophene-based HILs, another aspectof the present technology provides non-aqueous, organic solvent-basedink compositions formulated for inkjet printing HILs or HTLs for OLEDs.The organic HIL/HTL ink compositions comprise a component that isconventionally viewed as a wetting agent, but that is incorporated intothe HTL inks in carefully controlled quantities, such that it actuallyprevents the uncontrolled spreading and pixel cell spill-over that canoccur as a result of wetting. In some embodiments, the organic inkscomprise: (1) a hole injection material or a hole transporting material;(2) one or more organic solvents that solubilize the hole injection orhold transporting material; and (3) a fluorosurfactant. The holeinjection or hole transporting material is typically present in anamount of no greater than about 5 wt. %, more typically no greater than2 wt. % and still more typically no greater than about 1 wt. % (e.g.,from about 0.1 to about 1 wt. %), based on total weight of the inkcomposition. Organic solvents typically account for about 95 to about99.8 wt. % of the ink composition. The fluorinated surfactants aretypically present in amounts of no greater than about 0.15 wt. %. Forexample, in some embodiments of the organic solvent-based inkcompositions the fluorinated surfactants are present in amounts rangingfrom about 0.03 wt. % to about 0.1 wt. %.

Suitable hole injection materials for the organic solvent-based inkcompositions include polythiophenes, as described above. Suitable holetransporting materials include polyvinyl carbazoles or derivativesthereof, polysilanes or derivatives thereof, polysiloxane derivativeshaving an aromatic amine at the side chain or the main chain, pyrazolinederivatives, arylamine derivatives, stilbene derivatives,triphenyldiamine derivatives, polyaniline or derivatives thereof,polythiophene or derivatives thereof, polyarylamines or derivativesthereof, polypyrroles or derivatives thereof, poly(p-phenylenevinylene)or derivatives thereof, or poly(2,5 thienylene vinylene) or derivativesthereof.

Suitable organic solvents for the HIL/HTL ink compositions includealkoxy alcohol, alkyl alcohol, alkyl benzene, alkyl benzoate, alkylnaphthalene, amyl octanoate, anisole, aryl alcohol, benzyl alcohol,butyl benzene, butyrophenone, cis-decalin, dipropylene glycol methylether, dodecyl benzene, mesitylene, methoxy propanol, methylbenzoate,methyl naphthalene, methylpyrrolidinone, phenoxy ethanol,1,3-propanediol, pyrrolidinone, trans-decalin, valerophenon, andmixtures thereof.

The fluorosurfactants are surfactants comprising a fluorinated alkylchain. E. I. du Pont de Nemours and Company (Wilmington, Del.) sellsfluorinated surfactants under the tradenames Capstone and Zonyl. Thefluorosurfactants may be, for example, fluorotelemers (e.g., telomere Bmonoether with polyethylene glycol or 2-perfluoroalkyl) ethanol).Commercially available fluorosurfactants include Zonyl® FS 1033D, Zonyl®FS 1176, Zonyl® FSG, Zonyl® FS-300, Zonyl® FSN, Zonyl® FSH, Zonyl® FSN,Zonyl® FSO, Zonyl® FSN-100, Zonyl® FSO-100, Zonyl® FSH, Zonyl® FSN,Zonyl® FSO, Zonyl® FSH, Zonyl® FSN, Zonyl® FSO, Zonyl® FS 500, Zonyl® FS510, Zonyl® FSJ, Zonyl® FS-610, Zonyl® 9361, Zonyl® FSA, FSP, FSE, FSJ,Zonyl® FSP, Zonyl® 9361, Zonyl® FSE, Zonyl® FSA, Zonyl® UR, Zonyl®8867L, Zonyl® FSG, Zonyl® 8857A, Foraperle® 225, Forafac® 1268, Forafac®1157, Forafac® 1183, Zonyl® 8929B, Zonyl® 9155, Zonyl® 9815, Zonyl®9933LX, Zonyl® 9938, Zonyl® PFBI, Zonyl® PFBEI, Zonyl® PFBE, Zonyl®PFHI, Zonyl® BA, -8-Zonyl® PFHEI, Zonyl® TM, Zonyl® 8932, Zonyl® 7910,Zonyl® 7040, Foraperle® 321/325, Zonyl® 9464, Zonyl® NF, Zonyl® RP,Zonyl® 321, Zonyl® 8740, Zonyl® 225, Zonyl® 227, Zonyl® 9977, Zonyl®9027, Zonyl® 9671, Zonyl® 9338, and Zonyl® 9582, Capstone® ST-500,Capstone® ST-300, Capstone® ST-200, Capstone® ST-110, Capstone® P-640,Capstone® P-623, Capstone® P-620, Capstone® P-600, Capstone® FS-10,Capstone® FS-17, Capstone® FS-22, Capstone® FS-30, Capstone® FS-31,Capstone® FS-3100, Capstone® FS-34, Capstone® FS-35, Capstone® FS-50,Capstone® FS-51, Capstone® FS-60, Capstone® FS-61, Capstone® FS-63,Capstone® FS-64, Capstone® FS-64, Capstone® FS-65, Capstone® FS-66,Capstone® FS-81, Capstone® FS-83, Capstone® LPA, Capstone® 1460,Capstone® 1157, Capstone® 1157D, Capstone® 1183, Capstone® CPS,Capstone® E, Capstone® LMC, Capstone® CP, Capstone® PSB, Capstone® 4-I,Capstone® 42-I, Capstone® 42-U, Capstone® 6-I, Capstone® 62-AL,Capstone® 62-I, Capstone® 62-MA, Capstone® TC, Capstone® TR, andCapstone® TS.

EXAMPLES Example 1 Effect of Methicone on in-Pixel Uniformity

The following example illustrates the contact line pinning effect, andresulting improvement in luminescence uniformity, provided by methiconein an HIL inkjet ink composition.

Materials and Methods

Preparation of HIL Ink Compositions:

HIL ink compositions A and B were prepared with components andconcentrations shown in Table 1. Both Compositions A and B includemethicone in concentrations indicated. As a comparative example, an inkcomposition including the ingredients listed in Table 2, but lackingmethicone, was prepared (Comparative Composition).

TABLE 1 Composition A Composition B Ingredient Wt. % Wt. % PEDOT 34 34H₂O 36.9 35.97 Dimethyl propylene glycol 16 19 methylether (DPGME)1,3-propanediol 13 11 Methicone (Botanisil S18) 0.1 0.03 Viscosity [cP]11.1 13.60 ST [Dyne/cm] 43.4 43

TABLE 2 Comparative Composition Ingredient Wt. % PEDOT 34 H₂O 36Dimethyl propylene glycol 16 methylether (DPGME) 1,3-propanediol 13Methicone (Botanisil S18) 0 Viscosity [cP] 11.1 ST [Dyne/cm] 43.4

The ink compositions were formulated by placing a clean vial on abalance and transferring the desired amount of Botanisil S-18 into thevial using a Pasteur pipette. The balance was tared and the1,3-propanediol, water and DPGME were sequentially pipetted into thevial. The vial was then removed from the balance, capped and rotated tomix the resulting aqueous solution. The vial was then returned to thebalance and the desired quantity of a PEDOT dispersion (Haraeus CleviosTM PVP A1 4083) was pipetted into the vial. The vial was then removedfrom the balance, capped and rotated to mix the PEDOT with the othercomponents of the mixture. The resulting PEDOT ink composition was thenfiltered with a polytetrafluoroethylene (PTFE) filter membrane (2.0 μm)and the filtered composition was collected in an amber bottle. Finally,the bottle was sonicated for 15 minutes prior to use.

The comparative ink composition was made using the same procedurewithout the Botanisil S-18.

Viscosity and Surface Tension Measurements:

Viscosity measurements were carried out using a DV-I Prime Brookfieldrheometer. Surface tension was measured with a SITA bubble pressuretensiometer. The measured values for the methicone-containing inkcompositions A and B and the comparative ink composition (ComparativeComposition) are provided in Tables 1 and 2.

HIL Inkjet Printing and OLED Fabrication:

The HIL ink compositions were printed onto ITO anodes in OLEDarchitectures. The substrate of the OLED was glass, with a thickness of0.5 mm, on which an anode of 60 nm ITO (indium tin oxide) was patterned.A bank material (also known as a pixel definition layer) was thenpatterned over the ITO, forming a cell into which the inkjet printedlayers were deposited. The bank material was a negative workingphotoresist designed for inkjet printing. The resulting cells had bankswith heights in the range from about 0.5 to 2 μm that were angled at 45°with respect to the floor of the cell, such that the opening of eachcell was wider than its base. The 45° angle is representative of typicalbank angles, which range from about 5° to about 70°. The width andlength dimensions of the cells were about 60×175 μm. An HIL layer wasthen ink-jet printed, using the ink compositions of Tables 1 and 2, intothe cell, dried under vacuum and baked at an elevated temperature inorder to remove the water and solvents from the layer.

The HIL ink composition was printed at room temperature using the inkjetprinting system described in PCT application publication no. WO2013/158310, the entire disclosure of which is incorporated herein byreference. Inkjet printing into the pixel cells was carried out byfilling a bulk ink reservoir with the HIL ink composition. The bulk inkreservoir was in fluid communication with a primary dispensing reservoirand a continuous supply of the HIL ink composition was provided to theprimary dispensing reservoir during printing. The HIL ink compositionwas then fed into a printhead comprising a plurality of nozzles throughwhich the HIL ink composition was jetted into the pixel cells. Typicaldrop volumes during printing were about 10 pl and about 3 to 10 dropswere printed into each cell to form a droplet of the ink composition inthe cell.

An OLED incorporating an HIL that is printed from the Ink Composition Ais fabricated as follows. An HTL layer is inkjet printed onto the HILlayer, followed by drying under vacuum and baking at an elevatedtemperature to remove solvent and induce the crosslinking in thecrosslinkable polymer. Then the EML layer is inkjet printed onto the HTLlayer, followed by drying under vacuum and baking at an elevatedtemperature to remove solvent. The HTL and EML layers are inkjet printedusing the printer described above. The HTL ink composition is comprisedof a hole transporting polymer material in an ester-based solvent systemcomposed of a mixture of distilled and degassed diethyl octanoate andoctyl octanoate in a weight ratio of 1:1. The EML ink composition iscomprised of an organic electroluminescent material in diethyl sebacate.

An ETL layer, followed by a cathode layer, was then applied by vacuumthermal evaporation. The ETL material comprised lithium quinolate (LiQ)as an emissive material and the cathode layer was composed of 100 nm ofaluminum.

Results

Droplets of Compositions A and B printed into pixel cells were pinned tothe pixel banks and experienced neither spill-over nor pull-back. Animage of an HIL layer made using Composition A (0.1 wt. % methicone)printed in, and pinned to, a pixel cell is shown in FIG. 4A. Incontrast, images of the Comparative Composition printed into pixel cells(FIGS. 5 and 6) show that in the absence of methicone, the inkcomposition spread uncontrollably and spilled over 500 the sides of thepixel cell (FIG. 5A) or pulled back from the banks of the pixel cell(de-wetting) creating de-wetted areas 602 on the floor of the cellcombined with some pixel cell spill over 600 (FIG. 6A).

For each microphotograph shown in FIGS. 4A-6A, described above, andshown in FIGS. 7A-9A, described below, a black and white line drawing isprovided and labeled as the corresponding ‘B’ figure.

The electroluminescence properties of an OLED pixel including an inkjetprinted HIL made with Composition A was also investigated. Once the OLEDwas fabricated, the uniformity of its electroluminescence wasinvestigated by applying an electrical current across the diode andimaging the light emission. The resulting luminescence is shown in thephotomicrograph of FIG. 7A. As can be seen in that figure, the HIL layerprinted with Ink Composition A contributed to the uniform luminescenceof the OLED pixel into which it was incorporated.

Example 2 Effect of Sulfolane on Printing Properties

The following example illustrates the improved printing propertiesimparted to the HIL ink compositions by sulfolane.

Materials and Methods

Preparation of HIL Ink Composition:

A HIL ink composition including methicone, sulfolane and the otheringredients listed in Table 3 was prepared.

TABLE 3 Ingredient Wt. % PEDOT 59.9 DPGME 5 Sulfolane 10 H₂O 25Surfactant (S18) 0.1 Viscosity [cP] 5.9 ST [Dyne/cm] 45.0

The ink composition was formulated as described in Example 1, exceptthat sulfolane was used in place of the 1,3-propanediol.

Viscosity and Surface Tension Measurements:

Viscosity and surface tension measurements were carried out as inExample 1.

HIL Inkjet Printing and OLED Fabrication:

The HIL ink compositions were printed and OLED pixels were formed forelectroluminescence testing as described in Example 1.

Latency Measurements:

Latency measurements for the inks were carried out using the inkjetprinting system described in PCT application publication no. WO2013/158310. The measurements were conducted by firing one nozzle andmeasuring 300 data points of volume, velocity, and directionality. Thenozzle was then idled for 30 minutes. After 30 minutes the nozzle wasrestarted and 300 more data points were recorded.

The data sets were plotted and compared to look for any starting effects(usually velocity drop and volume change) at the beginning of the secondset of data (after the 30 minute idle) compared to steady-state jetting(end of the first data set, before the 30 minute idle).

Latency measurements for the inks were also carried out using a DimatixFujifilm DMP-2831 printer. In the drop watching setting all 16 nozzleswere turned on and it was confirmed that all nozzles were firing.Jetting was then stopped for 5 min. Jetting was resumed and inspectionconfirmed that all nozzles were still working. Then, continuous jettingwas carried out for periods of 15 and 30 min. Latency time was measuredas the time between the end of jetting and the onset of the drying ofthe ink in the uncapped nozzle, which results in improper dropletfiring. To determine when the ink compositions dried they were inspectedunder a microscope in white light and fluorescent modes.

Results

Once the OLEDs were fabricated, the uniformity of theirelectroluminescence was investigated by applying an electrical currentacross each diode and imaging the light emission. Electroluminescencewas measured for the OLED having an HIL printed from the ink compositionof Table 3 and for the OLED having an HIL printed from the inkcomposition of Table 1. A comparison of the microphotographs of FIGS. 8and 9, shows that sulfolane in the HIL ink composition provides a moreuniform pixel luminescence (FIG. 8A) than does propanediol (FIG. 9A).

In addition, the maximum stable jetting frequency for thesulfolane-containing ink composition (1000 Hz) was higher than that forthe diol-containing ink composition. Finally, the latency time for thesulfolane-containing ink composition was over 30 minutes, compared toonly 15 minutes for the diol-containing ink composition. The results forthe latency tests measured using the inkjet printing system described inPCT application publication no. WO 2013/158310, are shown in FIGS. 10 to12. In these graphs, the sulfolane-containing ink is designated P113.FIG. 10 is a graph of drop volume over 14 minutes for the inkcomposition before idle and after a 30 minute idle. FIG. 11 is a graphof drop velocity over 14 minutes for the ink composition before idle andafter a 30 minute idle. As can be seen in this figure, the drop velocityat restart was only 4% lower than the drop velocity before the idle.FIG. 12 is a graph of drop angle over 14 minutes for the ink compositionbefore idle and after a 30 minute idle. No significant difference indrop angle before and after idle was observed.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method of forming a hole injecting layer for anorganic light emitting diode, the method comprising: inkjet printing adroplet of an ink composition over an electrode layer in a pixel cell ofan organic light emitting diode, the pixel cell defined by a pixel bank,the ink composition comprising: an electrically conductive polythiophene(PEDOT); water; at least one organic solvent; and a methicone, whereinthe methicone is present in an amount that provides contact line pinningof the droplet in the pixel cell; and allowing the volatile componentsof the ink composition to evaporate, whereby the hole injecting layer isformed.
 2. The method of claim 1, wherein the electrically conductivepolythiophene is PEDOT.
 3. The method of claim 2, wherein the PEDOT ispresent in an amount of at least 50 wt. %.
 4. The method of claim 2,wherein the electrode layer comprises a transparent electricallyconductive material.
 5. The method of claim 4, wherein the transparentelectrically conductive material is indium tin oxide and the methiconeis present in an amount from about 0.03 wt. % to about 0.12 wt. %. 6.The method of claim 1, wherein the at least one organic solvent is anaprotic solvent having a surface tension no greater than 55 dyne/cm anda viscosity no greater than 15 cPs at 25°.
 7. The method of claim 6,wherein the at least one organic solvent has a boiling point atatmospheric pressure of 240° C. or higher.
 8. The method of claim 1,wherein the at least one organic solvent is sulfolane.
 9. The method ofclaim 8, wherein sulfolane is the majority organic solvent in the inkcomposition.
 10. The method of claim 8, wherein the sulfolane is presentin an amount of at least 5 wt. %.
 11. The method of claim 3, wherein theelectrode layer comprises indium tin oxide, the methicone is present inan amount from about 0.05 wt. % to about 0.1 wt. %, and the at least oneorganic solvent is sulfolane, which is present in an amount in the rangefrom about 5 to about 12 wt. %.
 12. The method of claim 8, wherein theink composition further comprises a second organic solvent having alower surface tension and a lower boiling point than sulfolane.
 13. Themethod of claim 12, wherein the second organic solvent is propyleneglycol methyl ether.
 14. The method of claim 1, wherein the inkformulation has a surface tension of no greater than 47 dyne/cm at 25°C., a viscosity of no greater than 15 cPs at 25° C. and a latency timeof at least 20 minutes at 25° C.
 15. An ink composition comprising:polythiophene (PEDOT); water; at least one organic solvent having asurface tension no greater than 55 dyne/cm at 25° C., a viscosity nogreater than 15 cPs at 25° C., and a boiling point of at least 200° C.;and a methicone.
 16. The ink composition of claim 15, wherein the atleast one organic solvent is sulfolane.
 17. The ink composition of claim16, further comprising a second organic solvent having a lower surfacetension and a lower boiling point than sulfolane.
 18. The inkcomposition of claim 17, wherein the second organic solvent is propyleneglycol methyl ether.
 19. The ink composition of claim 17, comprising:from about 50 to about 70 wt. % PEDOT; from about 3 wt. % to about 10wt. % sulfolane; and from about 0.3 to 0.12 wt. % methicone.